Various sensors capable of detecting the presence of a specific component (e.g., hydrocarbons, oxygen or carbon monoxide) contained in a gas have been developed for such purposes as the prevention of disasters, effective operation of machines or plants, and the like. However, unlike such gases as hydrocarbons, oxygen and carbon monoxide, carbon dioxide is chemically stable and, therefore, it is difficult to detect with sufficient sensitivity with a gas sensor utilizing hitherto known principles, e.g., an adsorption reaction or a combustion reaction of such a gas (see, e.g., U.S. Pat. No. 4,343,768).
It is known that carbon dioxide gas generates, when dissolved into water, hydrogen ions in proportion to the quantity dissolved therein and, hence, the concentration of carbon dioxide gas can be measured indirectly by measuring the concentration of hydrogen ion by use of a pH meter and a glass electrode (see U.S. Pat. No. 4,376,681). This method, however, requires a long period of time to dissolve carbon dioxide contained in a sample gas into water and remove it therefrom. In addition, the measurement tends to be strongly influenced by the presence of such foreign gases as SO.sub.x, NO.sub.x and NH.sub.3, which also could change the pH of the aqueous solution to be measured.
There is also known a method for detecting carbon dioxide gas, in which a sample of gas or fluid containing carbon dioxide gas, bicarbonate ion and/or carbonate ion is allowed to come into contact with an acid extracting fluid; a carbon dioxide-free gas is passed through the fluid in order to carry the dissolved carbon dioxide gas onto a carbon dioxide absorbing tube provided with an alkaline solution with a resulting change in the electrical conductivity of the alkaline solution, thus making it possible to measure the concentration of carbon dioxide gas contained in the sample (see U.S. Pat. No. 4,321,545). However, this method, like the above method using a pH meter, not only requires a long period of time for dissolving and removing carbon dioxide gas, but also is unable to distinguish the kind of ions detected. In addition, an apparatus to be used for the measurement could hardly be small in size.
There is also known a method utilizing the characteristic absorption of carbon dioxide gas in the infrared region of the spectrum. In general, a sensor utilizing this method consists of an IR ray generation section from which an IR beam with a wavelength of 4.25 .mu.m is emitted, a cell having a path length of several meters, an IR detector, and a fan which draws air through the cell. An apparatus utilizing the method, therefore, is expensive and could hardly be small in size. In addition, measurements utilizing the method are susceptible to the influence of dusts and other contaminants.
It is, therefore, desired, to develop a small and light carbon dioxide gas sensor capable of detecting the gas with a high accuracy and a quick response, without being influenced by dusts or the like.
In view of the above objective, the present inventors have conducted intensive investigations and found that carbon dioxide gas can be detected by utilizing a hydroxyapatite, which so far is known to be a porous ceramic usable as a moisture sensor since its electrical resistance changes in response to the change in moisture (see Japanese Patent Application (OPI) No. 166,249/83). (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) The moisture sensor of a hydroxyapatite utilizes the physical phenomenon that water absorbed on the surface of a hydroxyapatite penetrates into the pores of the porous ceramic and condenses therein. On the other hand, it has now been found that when a hydroxyapatite is brought into contact with carbon dioxide gas, carbonate apatite is formed therefrom in proportion to the concentration of carbon dioxide gas and, hence, the carbon dioxide gas can be detected through measurement of the change in its electrical resistance because the thus formed carbonate apatite has a greater electrical resistance than the hydroxyapatite. The present invention has been accomplished based on the above finding.